(704) Interamnia, (779) Nina, (330825) 2008 XE3, and 2012 QG42 and Laboratory Study of Possible Analog Samples ⇑ Vladimir V
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Icarus 262 (2015) 44–57 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Spectrophotometry of (32) Pomona, (145) Adeona, (704) Interamnia, (779) Nina, (330825) 2008 XE3, and 2012 QG42 and laboratory study of possible analog samples ⇑ Vladimir V. Busarev a,b, , Sergey I. Barabanov b, Vyacheslav S. Rusakov c, Vasiliy B. Puzin b, Valery V. Kravtsov a,d a Lomonosov Moscow State University, Sternberg Astronomical Institute, University Avenue 13, 119992 Moscow, Russia b Institute of Astronomy of Russian Academy of Science, Pyatnitskaya St. 48, 109017 Moscow, Russia c Division of Mossbauer Spectroscopy, Physical Dep. of Lomonosov Moscow State University, 119992 Moscow, Russia d Departamento de Fisica, Facultad de Ciencias Naturales, Universidad de Atacama, Copayapu 485, Copiapo, Chile article info abstract Article history: Six asteroids including two NEAs, one of which is PHA, accessible for observation in September 2012 were Received 3 April 2015 investigated using a low-resolution (R 100) spectrophotometry in the range 0.35–0.90 lm with the aim Revised 3 July 2015 to study features of their reflectance spectra. A high-altitude position of our Terskol Observatory (3150 m Accepted 4 August 2015 above sea level) favorable for the near-UV and visible-range observations of celestial objects allowed us Available online 8 August 2015 to probably detect some new spectral features of the asteroids. Two subtle absorption bands centered at 0.53 and 0.74 lm were found in the reflectance spectra of S-type (32) Pomona and interpreted as signs of Keywords: presence of pyroxenes in the asteroid surface matter and its different oxidation. Very similar absorption Asteroids bands centered at 0.38, 0.44 and 0.67–0.71 lm have been registered in the reflectance spectra of (145) Spectrophotometry Mineralogy Adeona, (704) Interamnia, and (779) Nina of primitive types. We performed laboratory investigations of ground samples of known carbonaceous chondrites, Orguel (CI), Mighei (CM2), Murchison (CM2), Boriskino (CM2), and seven samples of low-iron Mg serpentines as possible analogs of the primitive asteroids. In the course of this work, we discovered an intense absorption band (up to 25%) centered at 0.44 lm in reflectance spectra of the low-Fe serpentine samples. As it turned out, the equivalent width of the band has a high correlation with content of Fe3+ (octahedral and tetrahedral) in the samples. It may be considered as a confirmation of the previously proposed mechanism of the absorption due to elec- tronic transitions in exchange-coupled pairs (ECP) of Fe3+ neighboring cations. It means that the absorp- tion feature can be used as an indicator of ferric iron in oxidized and hydrated low-Fe compounds on the surface of asteroids and other atmosphereless celestial bodies. Moreover, our measurements showed that the mechanism of light absorption is partially or completely blocked in the case of intermediate to high iron contents. Therefore, the method cannot probably be used for quantitative estimation of Fe3+ content on the bodies. Based on laboratory study of the analog samples, we conclude that spectral characteristics of Adeona, Interamnia, and Nina correspond to a mixture of CI–CM-chondrites and hydrated silicates, oxi- des and/or hydroxides. Spectral signs of sublimation activity on Adeona, Interamnia, and Nina at minimal heliocentric distances are likely discovered in the short-wavelength range (0.4–0.6 lm). It is suggested that such cometary-like activity at the highest surface temperatures is a frequent phenomenon for C and close type asteroids including considerable amounts of ices beneath the surface. A usual way of origin of a temporal coma of ice particles around a primitive asteroid is excavated fresh ice at recent impact event (s). The obtained reflectance spectra of two NEAs, (330825) 2008 XE3 and 2012 QG42, are predominantly featureless and could be attributed to S(C) and S(B)-type bodies, respectively. We discuss reasons why weak spectral features seen in reflectance spectra of the main-belt asteroids are not observed in those of NEAs. Ó 2015 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Lomonosov Moscow State University, Sternberg Astronomical Institute, University Avenue 13, 119992 Moscow, Russia. E-mail address: [email protected] (V.V. Busarev). http://dx.doi.org/10.1016/j.icarus.2015.08.001 0019-1035/Ó 2015 Elsevier Inc. All rights reserved. V.V. Busarev et al. / Icarus 262 (2015) 44–57 45 1. Introduction calibration of the spectra was done using the positions of hydrogen Balmer lines in the spectrum of a Peg (B9III) observed in a repeated Spectrophotometry/spectroscopy is a traditional method of mode. The total exposure time spent on each target was typically h remote study of asteroids and other atmosphereless celestial bod- 1–2 . The obtained reflectance spectra were corrected for the dif- ies (e.g., McCord et al., 1970; Adams, 1974). When ground-based ference in air mass by applying a conventional method based on telescopes used, the range preliminary from 0.38 to 1.1 lmis using observations of a solar analog star (e.g., McCord et al., extended up to 2.5 lm (e.g., Vernazza et al., 2008; DeMeo et al., 1970). In our work a single solar analog star, HD 10307 (G1.5V) 2009; Hardersen et al., 2014; Fieber-Beyer et al., 2015). It is defined (Hardorp, 1980), was intentionally exploited to avoid possible dif- by the boundaries of the most transparent spectral ‘‘window” of ferences in the calculated reflectance spectra of asteroids as in the the Earth’s atmosphere, through which the bulk of observational case of several solar analogs use. Observations of the same star information on asteroids was obtained. It allowed us to enrich were performed to determine the running spectral extinction func- our knowledge about these objects, in particular on their taxonomy tion of the terrestrial atmosphere (Busarev, 2011). The observa- (e.g., Tholen, 1989; Bus and Binzel, 2002a, 2002b; DeMeo et al., tions of HD 10307 were made nearly in the same range of the air 2009). Further progress in ground-based studies of asteroids are masses at (or close to) which the asteroids of the sample were naturally related with increasing both the number (and therefore observed (see Table 1). The values of the signal-to-noise ratio the sample size) of studied bodies and the accuracy of spectral (S/N) of the asteroid spectra were estimated in the range of measurements. High-quality reflectance spectra of asteroids 0.4–0.8 lm. They are given, along with other data, in Table 1.To potentially contain not only valuable mineralogical information reduce high-frequency fluctuations in the reflectance spectra, they on the material of which the asteroids are made but also that on were smoothed by the method of ‘‘running box average” with a the valency state of iron (as well as of other transition metals). 5-point averaging interval. This allowed us to study considerably As is known, the latter depends on physico-chemical parameters wider spectral features of the observed asteroids and to assess (Platonov, 1976; Burns, 1993) of asteroid matter connected with their spectral types according to shapes of their reflectance spectra. the formation conditions of the bodies and their subsequent evolu- As a rule, averaging of the asteroid consecutive reflectance spectra tion. Unfortunately, various distorting factors, such as observa- was made when they had a close overall shape and observational tional faults and space weathering, make it difficult to spectra were obtained at minimal air masses. reconstruct reliably the previous conditions. Thus, the final goal Ephemerides (taken from the IAU Minor Planet Center on-line of such kind of study is to accurately extract the observational service at http://www.minorplanetcenter.net/iau/MPEph/MPEph. information and to try to correctly interpret it. For most asteroids, html) and observation parameters of the asteroids are given in except for some bodies investigated by space methods, there is a Table 1. lack of data about whether or not chemico-mineralogical and other properties vary along their surface. The reason of that is mainly due to (nearly) point-like appearance of asteroids at ground- 3. Analysis and interpretation of asteroid reflectance spectra based observations, which makes it difficult to obtain spectral information of different parts of the asteroids’ surface. Indeed, 3.1. 32 Pomona the angular size of (1) Ceres, the largest asteroid with a diameter of 1000 km, varies in the range 0.008–0.003, which is comparable Average diameter and geometric albedo of Pomona according to to the limiting angular resolution of ground-based telescopes at recent WISE-data are of 81.78 km and 0.25 (Masiero et al., 2014). h the excellent atmospheric seeing. The most important is to mini- The asteroid rotates with a period of 9.448 (Harris et al., 2012). mize the impact of the Earth’s atmosphere on reflectance spectra In total, eight separate spectra of Pomona were registered on the of asteroids and on the reliability of final results and conclusions. night 19/20 of September, 2012, together with the spectra of To achieve this goal it is useful to compare spectral data obtained HD10307 used as a solar analog star (Table 1). We used them to with the same facility on asteroids of the same and/or close taxo- calculate an average spectrum of the asteroid and normalized it nomic types which are expected to have similar spectral features. to 1.0 at 0.55 lm(Fig. 1a). It corresponds to an S-type body having Such approach is used in the work to study several of C-B-type mineralogy dominated by pyroxenes, olivines, and other high- asteroids. As before, a laboratory study of spectral characteristics temperature compounds (e.g., Gaffey et al., 1989, 1993).